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Partly cloudy or mostly sunny? Weather jargon, decoded

Derrick Snyder
10 a.m. EDT August 28, 2014

A low-hanging “scud” —a rapidly moving, wispy cloud on the leading edge of some thunderstorms — reaches down over the Indian River just north of Post Road and U.S. 1 in Melbourne, Florida on June 3, 2010.
(Photo:
Andrew Knapp/Florida Today
)

Many meteorologists, your author included, are guilty of using jargon and acronyms to describe weather phenomena. We meteorologists employ these terms as a means of saving time.

However, what is the point making a weather forecast if your audience thinks you are speaking a different language? I asked my non-meteorologist friends what commonly used weather terms confuse them. Let’s walk through some of these puzzling weather terms and abbreviations.

Cloud confusion

Terminology describing how much sky is covered by clouds has long been perplexing. Everyone knows what terms like “clear” or “overcast” mean.

The terms in between — partly cloudy, mostly sunny and mostly clouds — are more ambiguous. To assign the correct term to describe the condition of the sky, meteorologists divide the sky into eighths. Each one-eighth “slice” of the sky is called an okta.

A “clear” or “sunny” sky is one in which zero of a possible eight oktas of the sky is covered by cloud. A sky with one or two otkas of coverage is called mostly sunny (at night, the term used is mostly clear). If three to five oktas obscure the heavens, the sky is known as partly sunny, or partly cloudy if it is nighttime. When six to eight oktas of the sky are clouded over, mostly cloudy is the correct term. Finally, if all eight oktas are filled by cloud, the correct term is cloudy or overcast.

Storms or scud?

A number of cloud phenomena associated with thunderstorms are misreported. Three of the most frequently misidentified thunderstorm clouds are wall clouds, shelf clouds and scud.

According to the glossary of the American Meteorological Society, a wall cloud is a cloud that abruptly lowers from the base of a supercell thunderstorm. Wall clouds that exhibit strong rotation often precede the formation of tornadoes.

Shelf clouds are another type of low-level thunderstorm cloud, usually in an arc shape. They typically precede the arrival of a mature thunderstorm. Shelf clouds can be very dramatic and can cause panic among people unaccustomed to them. However, tornadoes are not produced by shelf clouds.

Scud clouds are frequently reported as tornadoes or simply as “rotation” and have prompted tornado warnings to be hoisted and sirens to be sounded without need. Scud clouds often scare the untrained eye because of their ragged appearance and proximity to the ground. In reality, scud clouds are formed by raindrops evaporating as they fall from a rain shaft or by water on the ground evaporating and condensing into clouds.